1. Field of the Invention
The present invention relates to a magnet, and more particularly, to a magnet which can generate magnetic flux to be more uniformly distributed around itself.
2. Description of the Related Art
Magnets are used in apparatuses using electromagnetic forces, such as optical pickup actuators, which are generally used in optical recording apparatuses to record and reproduce optical disks. In recent years, the size and compact disk access time of optical recorders have been reduced. Thus, optical pickup actuators are required to have a smaller size and provide higher disk-following speed. In addition, the optical pickup actuators are required to reproduce data from digital versatile disks (DVDs) as well as original compact disks (CDs).
To meet these requirements, the operational range of the optical pickup actuators must be expanded or maintained at a current level. In the meantime, non-uniformly distributed magnetic flux of a magnet, which is installed in an optical pickup actuator, may cause irregular thrust forces, i.e., irregular electromagnetic forces generated through interaction between the magnet and current, and irregular thrust forces may cause minor resonance of the optical pickup actuator. As the compact disk access time of the optical pickup actuator becomes reduced, the minor resonance phenomenon of the optical pickup actuator occurs more severely.
Here, the magnetic flux indicates that the number of magnetic lines that pass through the surface of a magnet, which is bounded by a closed loop, and the Si unit for the magnetic flux is tesla·meter2, or webers (Wb).
FIG. 1 is an exploded perspective view of a conventional asymmetric optical pickup actuator to which plate-shaped magnets are fixed, and FIG. 2 is a diagram illustrating the distribution of the magnetic flux of a general magnet. Referring to FIG. 1, an optical pickup actuator includes magnets 11; a yoke 13, to which the magnets 11 are fixed; a base 15, on which the yoke 13 is installed; a coil 12, which generates electromagnetic forces through interaction with the magnets 11; a bobbin 17, which allows the coil 12 to interact with the magnets 11 by coupling the base 15; an objective lens 19, which is installed on the bobbin 17; and a suspension 18, which supports the bobbin 17.
Referring to FIG. 2, the magnetic flux of each of the plate-shaped magnets 11 is concentrated at the center of the magnet 11. The closer to both ends of the magnet 11, the lower the density of the magnetic flux of the magnet 11, and thus the distribution of the magnetic flux of the magnet 11 at both ends of the magnet 11 becomes non-uniform.
As illustrated in FIG. 2, in the case of installing magnets showing non-uniform distribution of magnetic flux in the optical pickup actuator shown in FIG. 1, thrust forces are generated in a focusing direction as illustrated in FIG. 3, and a tracking direction as illustrated in FIG. 4. In FIGS. 3 and 4, the X-axis represents the direction of the width of the surfaces of the magnets 11 facing the coil 12 and the Y-axis represents the direction of the height of the surfaces of the magnet 11 facing the coil 12.
The focusing and tracking operations of the optical pickup actuator are performed by electromagnetic forces generated through interaction between the magnets 11 and the coil 12 installed in the bobbin 17. The electromagnetic forces can be expressed by Equation (1).{right arrow over (F)}={right arrow over (I)}L×{right arrow over (B)}  (1)Here, F represents electromagnetic forces, I represents current, L represents the length of the coil 12, at which current flows, and B represents the magnetic field. Here, the magnetic field, that is the magnetic flux density (B), indicates magnetic flux per unit area and is measured in Teslas. Here, 1T=1 Wb/m2.
Referring to FIG. 3, during operation of the optical pickup actuator, thrust forces in a focusing direction are stronger at the center of the magnet 11 than the other portions of the magnet 11. Here, the thrust forces represent electromagnetic forces generated between the coil 12 and the magnets 11 to follow a disk in the optical pickup actuator.
An assembly of the coil 12 and the bobbin 17 perform an on-focusing operation to read data from an optical disk and operate in a predetermined operational range in order to follow the optical disk that wobbles. During the disk-following operation, an assembly of the coil 12 and the bobbin 17 moves about the center of the magnet 11 in a vertical direction. When the assembly of the coil 12 and the bobbin 17 moves close to the center of the magnet 11, the maximum thrust forces are generated. On the other hand, when the assembly of the coil 12 and the bobbin 17 moves toward either end of the magnet 11, the minimum thrust forces are generated.
Referring to FIG. 4, when the assembly of the coil 12 and the bobbin 17 performs an on-tracking operation, the assembly of the coil 12 and the bobbin 17 operates in a predetermined operational area moving about the center of the magnet 11 in a horizontal direction in order to follow an optical disk that wobbles. During the on-tracking operation of the assembly of the coil 12 and the bobbin 17, the thrust forces in a tracking direction are stronger at the center of each magnet 11 than at the other portions of each magnet 11.
Thrust forces irregularly distributed in focusing and tracking directions cause the minor resonance phenomenon of the optical pickup actuator to occur. Here, the minor resonance phenomenon indicates a resonance phenomenon caused by the inherent frequencies of an optical disk and an actuator, and the minor resonance phenomenon of an optical disk means that the optical disk operates unstably.
In other words, the conventional magnet generates irregular thrust forces due to non-uniformly distributed magnetic flux, and the irregular thrust forces cause an optical pickup actuator to operate unstably.